Transfer Functions Description - ABB 2600T Series Operating Instructions Manual

12.4.1 Transfer functions description

Linear
Using this function, the relationship between the input
(measured value), expressed in % of the calibrated span and
the output is linear (i.e.: at 0% input, corresponds 0% output -
4mA, at 100% input corresponds 100% output - 20mA). No
further settings are possible here.
Square root
Using the Square Root function, the output (in % of the span) is
proportional to the square root of the input signal in percentage
of the calibrated span (i.e.: the instrument gives an analog
output proportional to the rate of flow).
The possibility to have the full Square Root function is given.
To avoid the extremely high gain error with the input
approaching zero, the transmitter output is linear with the input
up with a slope of 1 up to 0.5% and then still linear with the
appropriated slope to a programmable percentage value
between 10 % and 20%. This option is offer in order to ensure
a more stable output when the signal is close to zero avoiding
errors due to the high gain of the square root.
To neglect the values with the input approaching zero, the
transmitter output is zero with the input up to a programmable
percentage value between 0 % and 20%. This option is offer in
order to ensure a more stable flow measure. This option is
possible for all the listed output functions. ).vant operating
instruction.
Square root to the 3rd power
The x3 Square root Transfer function can be used for open
channel (see figures on the right) flow measurement using ISO
1438 rectangular weirs (Hamilton Smith, Kindsvater-Carter,
Rehbock formulas) or trapezoidal weirs (Cippoletti formulas)
and ISO 1438 Venturi flumes. In these types of devices the
relationship between the flow and the developed head h (the
differential pressure measured by the transmitter) is
proportional to h3/2 or square root of h3. Other types of Venturi
or Parshall flume do not follow this relationship. Using this
function, the output (in % of the span) is proportional to the
square root of the third power of the input signal in % of the
calibrated span: the instrument gives an output proportional to
the rate of flow calculated using the above mentioned formulas.
Square root to the 5th power
The x5 Square root Transfer function can be used for open
channel flow measurement using ISO 1438 Vnotch (triangular)
weirs (see figure on the right) where the relationship between
the flow and the developed head h (the differential pressure
measured by the transmitter) is proportional to h5/2 or square
root of h5.
Using this function, the output (in % of the span) is proportional
to the square root of the fifth power of the input signal in % of
the calibrated span: the instrument (it gives an output
proportional to the rate of flow calculated using the Kingsvater-
Shen formula).
44 OI/266/PB-EN Rev. E | 2600T Series Pressure transmitters
Custom linearization curve
The custom linearization curve transfer function it is used
typically for volumetric level measurement in tanks with an
irregular shape. It can be registered to a freely identifiable
transfer function with a maximum of 22 base points. The first
point is always the zero point, the last is always the final value.
Neither of these points can be altered.
A maximum of 20 points can be freely entered in between.
These points have to be defined by extrapolating the tank filling
table data and reducing them to 22 points.Once identified the
22 points they will need to be uploaded into the device by
either using an HART hand held terminal or a proper
configuration software like Asset Vision Basic.
Bidirectional flow (to be used when the transmitter is
connected to a bidirectional flow element)
Main characteristics
The bidirectional function, applied to the transmitter input (x)
expressed in percentage of the calibrated span, has the
following form:
Output = ½ + ½ sign (x) ∙ x ½
where "x" and "Output" should be normalized in the range 0 to
1 for calculation purpose, with the following Output meaning:
— Output = 0 means Analog out 4 mA;
— Output = 1 means Analog out 20 mA.
This function can be used for flow measurement purpose when
the flow is in both the directions and the primary elements are
designed to perform this type of measure. As an example, if we
have a bidirectional flow measurement application with the
following data:
Max reverse flow rate: -100 l/h
Max flow rate: +100 l/h
The differential pressure generated by the flow primary is for
the maximum flow rate 2500 mmH2O, for the max reverse flow
rate 2500 mmH2O. The transmitter will have to be configured
as follows:
Calibrated span:
Transfer function:
Once configured as above the transmitter will deliver:
flowrate 100 l/h reverse:
no flowrate:
Flow rate 100 l/h:
Cylindric lying tank
This function is used to measure the volumetric level into a
cylindrical horizontal tank with flat ends. The transmitter
calculates the volume from the measured filling level.
Spherical tank
This function is used to measure the volumetric level into a
spherical tank. The transmitter calculates the volume from the
measured filling level.
12 Operations
4mA = LRV = -2500mmH2O
20mA = URV = +2500mmH2O
Bidirectional flow
output= 4 mA
output= 12 mA
output= 20 mA